Without a doubt, the URL vs. URI post is by the most visited page on this blog. Even so, there’s still a lot of confusion on the topic and so I thought I’d break it down in less words. The original post was slightly misleading in that I attempted to compare URI to URL, when in fact it should have defined the relationship between URI, URL, and URN. In this post, I hope to clear that in more concise terms. But first, here’s a pretty picture:

uri_class_diagram

Both URLs and URNs are specializations, or subclasses of URI. You can refer to both URLs and URNs as a URI. In applictaion terms: if your application only calls for a URI, you should be free to use either or.
Now, here’s where the big difference between URN and URL: a URL is location bound and dereferencable over the web. A URN is just a name and isn’t bound to a network location. However, BOTH are still valid URIs. Now, if the application requires a URI that is bound to a network location, you must use the specialization of URI called URL.

Remember that URI stands for Uniform Resource Identifier, which is used to identify some “thing”, or resource, on the web. Both URLs and URNs are specialization’s (or subclasses if you will), of URI. You’d be correct by referring to both a URL or URN as a URI. In applictaion terms: if your application only calls for an identifier, you should be free to use either a URL or a URN. For brevity, you can state that the application simply requires a URI and the use of a URL or URN would statisfy that requirement.

Now if your application needs a URI that dereferencable over the web, you should be aware of the difference between URN and URL. A URL is location bound and defines the mechanism as to how to retrieve the resource over the web. A URN is just a name and isn’t bound to a network location. For example, you may have a URN for a books ISBN number in the form of urn:isbn:0451450523. The URN is still a valid URI, but you cannot dereference it, it’s just a name used to provide identity. So to put in simpler terms:

  • A URI is used to define the identity of some thing on the on the web
  • Both URL and URN are URIs
  • A URN only defines a name, it provides no details about how to get the resource over a network.
  • A URL defines how to retrieve the resource over the web.
  • You can get a “thing” via a URL, you can’t get anything with a URN
  • Both URL and URN are URIs as the both identify a resource

There some other items that need clarification based on some comments I’ve received from the original post:

  • Elements of a URI such as query string, file extension, etc. have no bearing on whether or not a URL is a URI. If the URI is defines how to get a resource over the web, it’s a URL.
  • A URL is not limited to HTTP. There are many other protocol schemes that can be plugged into a URL.
  • If a URL defines a scheme other than HTTP, it does not magically become a URI. The URI defines how to get the resource, whether it be HTTP, FTP, SMB, etc., it’s still a URL. But because the URL identifies a resource, it’s a URI as well.

Yeah, I’ve probably repeated myself a few times, but I wanted to stress a few points.

There’s also been some confusion about when to use the term URI. As I stated in the original post explained above, it depends on what you’re doing. If everything your application does involves accessing data over the web, you’re most likely using URL exclusively. In that case, it wouldn’t be a bad thing to use the term URL. Now, if the application can use either a network location, or a name, then URI is the proper term. For example, XML namespaces are usually declared using a URI. The namespace may just be a name, or a URL that references a DTD or XML Schema. So if you’re using a URL for identity and retrieval, it’s probably best to use URI.

Updated (1/18/2011) : Because there’s still a lot of confusion, I’ve created a third post that attempts to resolve a lot of the questions from the comments on the last two posts. The new post is here.

Updated (8/27/2009): I’ve created another post that attempts to make the distinctions a bit more clear. The new post is here.

What is the difference between a URL and URI and why does it matter? This topic is confusing to some (myself included) and I thought I’d share my understanding of the two concepts. I’m hoping this post will give you a better understanding about how the two differ and why it matters to some.

Note: The goal of this post is to simplify the distinction between URI and URI. If you feel that in the summarization process something was lost, or it’s simply just correct, please post a comment and the information will be corrected. I only ask for any comments/criticism to be constructive.

Update: Thanks some constructive, and not-so constructive, feedback from some readers I have updated this post to correct many of my own misunderstandings. Of which, there were many.

URI

A URI identifies a resource either by location, or a name, or both. More often than not, most of us use URIs that defines a location to a resource. The fact that a URI can identify a resources by both name and location has lead to a lot of the confusion in my opionion. A URI has two specializations known as URL and URN.

URN

A URI identifies a resource by name in a given namespace but not define how the resource maybe obtained. This type of URI is called a URN. You may see URNs used in XML Schema documents to define a namespace, usually using a syntax such as:

<xsd:schema xmlns="http://www.w3.org/2001/XMLSchema"
            xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            targetNamespace="urn:example"

Here the targetNamespace use a URN. It defines an identifier to the namespace, but it does not define a location.

URL

A URL is a specialization of URI that defines the network location of a specific resource. Unlike a URN, the URL defines how the resource can be obtained. We use URLs every day in the form of http://damnhandy.com, etc. But a URL doesn’t have to be an HTTP URL, it can be ftp://damnhandy.com, smb://damnhandy.com, etc.

The Difference Between Them

So what is the difference between URI and URL? It’s not as clear cut as I would like, but here’s my stab at it:

A URI is an identifier for some resource, but a URL gives you specific information as to obtain that resource. A URI is a URL and as one commenter pointed out, it is now considered incorrect to use URL when describing applications. Generally, if the URL describes both the location and name of a resource, the term to use is URI. Since this is generally the case most of us encounter everyday, URI is the correct term.